The invention relates to a shielding can for protecting components on a printed circuit board (PCB), preferably for a cellular phone, against interference from other components due to EMC. A shielding can is a cage that covers a number of electronic components on a printed circuit board, and thereby protects these components from disturbing other electronic components on the printed circuit board.
The use of shielding cans is well known because all GSM phones have printed circuit boards equipped with different components that disturb each other if they are not shielded from each other.
The shielding cans are usually made in at least two different embodiments. The first embodiment comprises one entire box shaped piece soldered onto the printed circuit board. This type of shielding can does not permit one to access components covered by the shielding can, which makes it very difficult to repair or inspect any of the components covered or shielded by the shielding can. Furthermore it is often required to solder the components beneath the shielding can onto the printed circuit board before soldering the shielding can, because you cannot get enough heat to the components to, make a good solder joint. An alternative is having holes or apertures to let the heat in to solder the components at the same time as the shielding can. However these types of shielding cans are often made of a rather thick material so even with holes or apertures you cannot get enough heat to solder the components properly at the same time as the shielding can.
The second embodiment includes two pieces, one frame and one lid. The lid is snapped onto the frame and can be removed from the frame, which is soldered onto the printed circuit board, to enable repairing or inspection of the components shielded by the shielding can. The second embodiment may be attached to the printed circuit board in different ways. One way is to solder the frame onto the printed circuit board first and then attach the lid to the frame. Another way is snap the lid onto the frame and solder the assembly onto the printed circuit board, but then there is a need for holes or apertures in the lid to let in the heat during soldering. Otherwise the components covered or shielded by the shielding can will not be properly soldered since the heat can not reach the components. It is preferable to solder the frame with the lid attached because you save work operations. However according to prior art there are problems getting a good solder joint, when using a pre-joined shielding can.
During the soldering process the printed circuit boards are moved continuously through an oven. When the printed circuit board leaves the oven a significant temperature gradient is generated and this causes the soldering to congeal from one end towards the other end. Due to internal stress, caused because one end of the shielding can is in a hot area while the other end is starting cooling off, in the shielding cans, they tend to start lifting in the hottest end prior to the congealing. The internal stress, causing the lifting, is a problem, when having rather thin material in the shielding can. To avoid these kinds of problems one has until now relied on: manual soldering, smaller shielding cans, thicker material in the shielding cans, a lower cooling gradient or soldering of the frame without the lid.
An object of the invention is to provide a shielding can for a printed circuit board.
This objective is obtained by a substantially rectangular shielding can, which include side walls or a frame and a lid adjacent to the side walls. The lid has means for increasing the flexibility of the lid. This flexibility is, according to the preferred embodiment, obtained by providing at least one slit that is adjacent to the sides of the lid and extending substantially transverse thereto. There ought to be at least two slits on the lid, one on each side of the lid that is exposed for a significant temperature gradient during the soldering process. There could be slits on the other sides to simplify the production of the shielding cans. Then the pick-and-place machine does not have to care about the orientation of the shielding can when placing the can on the printed circuit board.
Another object of the invention is to provide a communication terminal with a printed circuit board having shielding cans.
This is achieved through a communication terminal with a printed circuit board having shielding cans where said shielding cans are substantially rectangular and includes side walls and a lid adjacent to the side walls, where the lid has means for increasing the flexibility of the lid. The means for increasing the flexibility of the lid, according to the preferred embodiment, are slits on at least two sides of the lid, where the each slit is adjacent to the sides of the lid and extending substantially transverse thereto.
Yet another object of the invention is to provide an improved method of attaching a shielding can to a printed circuit board
This purpose is obtained by a shielding can, which comprise two parts, a frame with side walls and a lid, adjacent attached to the frame. The mounting of shielding cans is executed by e.g. a pneumatical device (pick and place machine) that places the shielding can onto a printed circuit board, which has been prepared with solder paste, in one joined piece. After that the printed circuit board is warmed to soldering the shielding can onto the printed circuit board, where the soldering temperature increases slowly to make the solder paste melt entirely. This is followed by a fast cooling phase, where the shielding can has means for increasing the flexibility of the lid, formed by slits on the sides of the lid, to adapt to the temperature changes.
With this method it is possible to handle and to mount the shielding can in one step and thereby reducing the working steps.
The invention will be explained more fully below, by way of example, in connection with preferred embodiments and with reference to the drawings, in which:
FIG. 1 schematically illustrates a preferred embodiment of a hand portable phone having a shielding can according to the invention.
FIG. 2 schematically shows the essential parts of a telephone for communication with a cellular network.
FIGS. 3a-b shows a printed circuit board having a shielding can according to the invention.
FIG. 4 shows a shielding can according to the invention.
FIG. 5 shows a frame according to the invention.
FIG. 6 shows a lid according to the invention.
FIG. 7 shows an overview of the mounting process of the shielding can onto the printed circuit board.
FIG. 8 shows a printed circuit board having a shielding can according to the invention.